The current state of understanding of molecular resonance energy transfer (RET) and recent developments in the field are reviewed. The development of more general theoretical approaches has uncovered some new principles underlying RET processes. This review brings many of these important new concepts together into a generalization of Forster's original theory. The conclusions of studies investigating the various approximations in Forster theory are summarized. Areas of present and future activity are discussed. The review covers Forster theory for donor-acceptor pairs and electronic coupling for singlet-singlet, triplet-triplet, and superexchange-mediated energy transfer. This includes the transition density picture of Coulombic coupling as well as electronic coupling between molecular aggregates (excitons). Spectral overlaps and ensemble energy transfer rates in disordered aggregates, the role of dielectric properties of the medium, weak versus strong coupling, and new models for energy transfer in complex molecular assemblies are also described.

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Long-Range Resonance Energy Transfer in Molecular Systems

[1] Methane gas hydrates, crystalline inclusion compounds formed from methane and water, are found in marine continental margin and permafrost sediments worldwide. This article reviews the current understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate-bearing sediments. Formation phenomena include pore-scale habit, solubility, spatial variability, and host sediment aggregate properties. Physical properties include thermal properties, permeability, electrical conductivity and permittivity, small-strain elastic P and S wave velocities, shear strength, and volume changes resulting from hydrate dissociation. The magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate-bearing sediments to changes in mechanical, thermal, or chemical boundary conditions. These predictions are vital for mitigating borehole, local, and regional slope stability hazards; optimizing recovery techniques for extracting methane from hydrate-bearing sediments or sequestering carbon dioxide in gas hydrate; and evaluating the role of gas hydrate in the global carbon cycle.

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Physical properties of hydrate-bearing sediments

“Green” path from fossil-based to hydrogen economy: An overview of carbon-neutral technologies

The template carbonization method is a powerful tool for producing carbon materials with precisely controlled structures at the nanometer level. The resulting templated nanocarbons exhibit extraordinarily unique (often ordered) structures that could never be produced by any of the conventional methods for carbon production. This review summarizes recent publications about templated nanocarbons and their composites used for energy storage applications, including hydrogen storage, electrochemical capacitors, and lithium-ion batteries. The main objective of this review is to clarify the true significance of the templated nanocarbons for each application. For this purpose, the performance characteristics of almost all templated nanocarbons reported thus far are listed and compared with those of conventional materials, so that the advantages/disadvantages of the templated nanocarbons are elucidated. From the practical point of view, the high production cost and poor mass-producibility of the templated nanocarbons make them rather difficult to utilize; however, the study of their unique, specific, and ordered structures enables a deeper insight into energy storage mechanisms and the guidelines for developing energy storage materials. Thus, another important purpose of this work is to establish such general guidelines and to propose future strategies for the production of carbon materials with improved performance for energy storage applications.

Templated nanocarbons for energy storage.

Investigation into gas production from natural gas hydrate: A review

Visual observation of gas hydrates at the microscopic scale in synthetic porous media provides unequivocal visual evidence that clathrates can form in systems without the presence of a free-gas phase. Hydrates were formed from a soluble liquid hydrate former (tetrahydrofuran, C 4 H 8 O), from free gas (CH 4 ), and from dissolved gas (CO 2 ). Clathrates were found to form within the center of pore spaces, rather than on grain surfaces. Cementation of grains only occurred in regions of a small grain size, or where a large proportion of pore space was filled with hydrate. However, even at high clathrate saturations, a thin film of free water persisted on grain surfaces. The results have important implications for the potential cementing effect of hydrates on sediments, and thus for sediment permeability, slope stability, and seismic interpretation of hydrate-bearing sediments.

Visual observation of gas-hydrate formation and dissociation in synthetic porous media by means of glass micromodels

Carbon monoxide occurs in abundance throughout the cosmos, potentially in clathrate form, whereas on Earth, it forms a notable constituent of industrial flue gases. It has been proposed that hydrate technology could be used in CO2 separation from flue gases, and in subsea flue gas CO2 disposal. This—and the likely widespread occurrence of CO clathrates in the cosmos—means it is important that the phase behavior of CO hydrates is known. Here, we present experimental H-L-V (hydrate–liquid–vapor) equilibrium data for CO, COCO2, and COC3H8 (propane) clathrate hydrates. Data were generated by a reliable step-heating technique validated using measured data for CO2 and CH4 hydrates. Data for CO and COC3H8 clathrates have been used in the optimization of Kihara potential parameters for CO, reported here, facilitating the extension of a thermodynamic model to the prediction of CO hydrate equilibria. Model predictions are validated against independent experimental data for COCO2 (structure I) systems, with good agreement being observed. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Carbon monoxide clathrate hydrates: Equilibrium data and thermodynamic modeling

We present experimental structure-I clathrate hydrate (methane, carbon dioxide, and methane−carbon dioxide) equilibrium and ice-melting data for mesoporous silica glass. In both cases, high capillary pressures result in depressed solid decomposition temperatures (clathrate dissociation and ice melting), as a function of pore diameter. Clathrate dissociation data show a significant improvement over existing literature data, which is attributed to the improved experimental techniques and interpretative methods used. Through application of a melting (or clathrate dissociation) modified Gibbs−Thomson relationship to experimental data, we determine similar values of 32 ± 2, 32 ± 3, and 30 ± 3 mJ/m2 for ice−water, methane clathrate−water, and carbon dioxide clathrate−water interfacial tensions, respectively. The data are important for the accurate thermodynamic modeling of clathrate systems, particularly with respect to subsea sedimentary environments, and should prove useful in the simulation of potential meth...

Experimental measurement of methane and carbon dioxide clathrate hydrate equilibria in mesoporous silica

Phase relations for tetra-n-butylammonium bromide (TBAB, (C4H9)4N+Br-) and tetra-n-butylammonium fluoride (TBAF, (C4H9)4N+F-) aqueous systems under hydrogen pressure reveal the formation of binary H2−TBAB and H2−TBAF semi-clathrate hydrates which are stable at ambient temperatures (≤29 °C) and atmospheric pressure. H2 is most likely accommodated by occupation of normally vacant dodecahedral (512) cavities, with preliminary analyses suggesting gas contents are an order of magnitude greater than that for binary H2−THF clathrates at the same pressure (1 MPa).

Low-pressure molecular hydrogen storage in semi-clathrate hydrates of quaternary ammonium compounds.

Experimentally determined equilibrium phase relations are reported for the system H 2 -THF-H 2 O as a function of aqueous tetrahydrofuran (THF) concentration from 260 to 290 K at pressures up to 45 MPa. Data are consistent with the formation of cubic structure-II (CS-II) binary H 2 -THF clathrate hydrates with a stoichiometric THF-to-water ratio of 1:17, which can incorporate modest volumes of molecular hydrogen at elevated pressures. Direct compositional analyses of the clathrate phase, at both low (0.20 mol %) and stoichiometric (5.56 mol %) initial THF aqueous concentrations, are consistent with observed phase behavior, suggesting full occupancy of large hexakaidecahedral (5 12 6 4 ) clathrate cavities by THF, coupled with largely complete (80-90%) filling of small dodecahedral (5 12 ) cages by single H 2 molecules at pressures of >30 MPa, giving a clathrate formula of (H 2 )≤2·THF·17H 2 O. Results should help to resolve the current controversy over binary H 2 -THF hydrate hydrogen contents; data confirm recent reports that suggest a maximum of ∼1 mass % H 2 , this contradicting values of up to 4 mass % previously claimed for comparable conditions.

Ross Anderson

Papers

Visual observation of gas-hydrate formation and dissociation in synthetic porous media by means of glass micromodels

Carbon monoxide clathrate hydrates: Equilibrium data and thermodynamic modeling

Experimental measurement of methane and carbon dioxide clathrate hydrate equilibria in mesoporous silica

Low-pressure molecular hydrogen storage in semi-clathrate hydrates of quaternary ammonium compounds.

Phase relations and binary clathrate hydrate formation in the system H2-THF-H2O.